"There is a view that in order to gain static assurances such as an array index being always in range or tail being applied to a non-empty list, we must give up on something significant: on data structures such as arrays (to be replaced with nested tuples), on general recursion, on annotation-free programming, on clarity of code, on well-supported programming languages.
That does not have to be the case. [These examples] show non-trivial examples involving native Haskell arrays, index computations, and general recursion. All arrays indexing operations are statically guaranteed to be safe -- and so we can safely use an efficient unsafeAt provided by GHC seemingly for that purpose. The code is efficient; the static assurances cost us no run-time overhead. The example uses only Haskell98 + higher-ranked types. No new type classes are introduced. The safety is based on: Haskell type system, quantified type variables, and a compact general-purpose trusted kernel."